The present invention concerns Rogowski coils. More particularly, the present invention concerns the structure and manner of making a Rogowski coil.
Rogowski coils are well known electrical devices finding use today for measurement of magnetic fields and electrical currents. They have been researched over the past century and are well known to the scientific literature. Their origin traces to the invention circa 1912 of the Rogowski coil by W. Rogowski and W. Steinhousen. The device is useful for measuring electrical currents and operates on the basis of a magnetic field integration performed across a closed contour being equal to the current flowing through the contour. The coil provides a voltage output proportional to the time derivative of the current (di/dt) rather than a current output like other current transformers.
Rogowski coils are popular because of their dynamic range and linearity. However, though theoretical requirements are known, manufacturers still need ways to provide a high quality coil that is both economical to manufacture and which is satisfactory for precise current measurements. The device (coil) should be insensitive to external influences, insensitive to the measured primary conductor position, and retain high precision (in the order of 0.3% or better) over its lifetime and across a wide temperature range (nominally −40 to 70 degrees Centigrade).
One known approach to making a Rogowski coil involves using a printed circuit. U.S. Pat. No. 5,414,400 entitled “Rogowski Coil” discloses a Rogowski coil made on a printed circuit plate provided with a circular cut-out. The coil is implemented by metal deposits on each of the two faces of the plate extending along radii, with electrical connections between the radii on one face and those on the opposite face being achieved via plated-through holes passing through the thickness of the plate.
U.S. Pat. No. 5,442,280 discloses a method for manufacturing a printed circuit board-based Rogowski coil. The disclosed geometry provides very high turn density resulting in very high sensitivity. While high sensitivity is very desirable when measuring low frequency currents (50/60 Hz power system related), the patent fails to provide adequate means for external field cancellation. This problem is reported in U.S. Pat. No. 6,624,624 and is caused by inadequate handling of the coil return path.
A similar problem applies to the design reported in U.S. Pat. No. 6,313,623 (by the current inventor) in which two closely spaced coils with counter rotation are used to perform partial return path compensation.
Further attempts to design precision Rogowski coils are disclosed in U.S. Pat. No. 6,624,624. Attempts to provide improved return path cancellation resulted in significantly reduced coil densities, making the design less appropriate for low frequency current measurement applications. In addition, although significantly improved, all reported geometries suffer from Z-axis (board thickness) related sensitivity contour effects with an error cancellation (return) path normally offset in the direction of the Z-axis (board thickness).
J. D. Ramboz in “Machinable Rogowski Coil, Design and Calibration,” IEEE Transactions on Instrumentation and Measurement, Vol. 45, No. 2, (April 1996) pp 511-15 reviews traditional designs for Rogowski coils and discusses a “machinable” Rogowski coil constructed using machinable ceramic material to make a toroidal coil with a rectangular cross section. A thin, electrically conductive coating is then applied to the coil, totally encapsulating the ceramic core. Next, thin lines of the conductive material are removed by laser machining methods in a pattern which leaves coils as bands of conductive material located radially around the core. Each turn or band was connected to the next turn by a suitable indexing.
U.S. Pat. No. 6,300,857 for “Insulating Toroid Cores and Windings” discloses a configuration to improve the winding of precise conventional transformer coils and includes an insulating jacket around a magnetic core. The insulating jacket includes plural protrusions around the core, the protrusions demarking various segments of the toroid. For example, the toroid may be divided into six evenly spaced sections, each occupying approximately 60°. At the edges of each section, there is a protrusion. The protrusions maintain the placement and spacing of windings within each section.
An object of the present invention is to provide a precision Rogowski coil with its winding geometry defined and controlled at the time of manufacture, with improved dimensional stability maintained throughout its lifetime.